1. Field of the Invention
This invention relates to an area cutting method in a numerically controlled machine tool and, more particularly, to an area cutting method for cutting an area delimited by a curve of an external shape and at least two closed curves which lie within the curve of the external shape.
2. Description of the Related Art
Forms of numerically controlled machining include cutting in which the interior of an area bounded by the curve of an external shape comprising straight lines and circular arcs is hollowed out down to a predetermined depth, and die milling in which the interior of an area is die milled. In such machining, as shown in FIG. 1(A), an area cutting method is conventionally carried out by performing cutting along an (i-1)th cutting path PT.sub.i-1 in one direction (the direction of the solid-line arrow), raising the tool a predetermined amount at the completion of cutting, then positioning the tool directly above a cutting starting point Ps on the next, or i-th, cutting path PT.sub.i by moving the tool in the direction of the dashed-line arrow, thereafter lowering the tool to the cutting starting point Ps, moving the tool along the i-th cutting path PT.sub.i in the direction of the solid line arrow, and subsequently repeating the above unidirectional cutting.
Another area cutting method shown in FIG. 1(B) includes, following completion of cutting along the cutting path PT.sub.i-1 of the (i-1)th cutting path, moving the tool from a cutting end point P.sub.e to the cutting starting point Ps on the next, or i-th, cutting path, and thereafter performing cutting along the i-th cutting path PTi. Thus, cutting is performed back and forth in the direction of the solid-line arrows.
There are cases where it is required to cut an area AR (shaded portion) delimited by a curve OLC of an external shape and a closed curve INC inside the curve of the external shape, as shown in FIG. 2. The reason is that it is necessary to form bolt holes, which are needed for effecting a connection to another part, in the area (hereafter referred to as an "island") IRD bounded by the closed curve INC, or to hollow out the island IRD to a depth less than that of the area AR by using a different tool.
Thus, if the island IRD is present inside the curve of the external shape, the area AR (the shaded portion in FIG. 2) delimited by the shape OLC of the external curve and the closed curve INC is cut, in accordance with the conventional method shown in FIG. 1(A) or 1(B), by (a) obtaining intersection points P.sub.i, Q.sub.i between a straight line SL.sub.i and the curve OLC (actually an offset curve) of the external shape, and obtaining intersection points R.sub.i, S.sub.i between the straight line SL.sub.i and the closed curve INC (actually an offset curve), (b) then moving the tool in a cutting-feed mode from the point P.sub.i to the point R.sub.i after positioning the tool at the point P.sub.i, which is a cutting starting point, (c) raising the tool along the +Z axis up to an approach plane APP (FIG. 3) after the tool arrives at the point R.sub.i, (d) moving the tool in a rapid-traverse mode on the approach plane, after the tool has arrived at the approach plane, to a point S.sub.i ' directly above the point S.sub.i, (e) then lowering the tool in the cutting-feed mode to the point S.sub.i, (f) thereafter moving the tool in the cutting-feed mode from the point S.sub.i to the point Q.sub.i, and (g) subsequently repeating steps (a) through (f) for the next cutting path.
Area cutting can thus be carried out with the conventional methods even if the island IRD is present. However, the conventional methods cannot be applied as they are if two or more islands are present and the spacing between the islands is small. By way of example, the conventional methods cannot be applied if, as shown in FIG. 4, two islands IRD.sub.1, IRD.sub.2 reside so close together inside the curve OLC of the external shape that offset curves INF.sub.1, INF.sub.2 intersect, the latter curves being obtained by offsetting closed curves INC.sub.1, INC.sub.2, which specify the respective islands, by a predetermined amount D (e.g., a value obtained by adding tool radius r.sub.a and finishing tolerance t). If the conventional methods are forced into use in this case, the tool TL will unfortunately cut into either island IRD.sub.1 or island IRD.sub.2 in the vicinity of the intersection CSA.